Colossus,
the first large-scale electronic computer, was used against the
German system of teleprinter encryption known at Bletchley Park as
‘Tunny’. Technologically more sophisticated than Enigma,
Tunny carried the highest grade of intelligence. From 1941 Hitler and
the German High Command relied increasingly on Tunny to protect their
communications with Army Group commanders across Europe. Tunny
messages sent by radio were first intercepted by the British in June
1941. After a year-long struggle with the new cipher, Bletchley Park
first read current Tunny traffic in July 1942. Tunny decrypts
contained intelligence that changed the course of the war in Europe,
saving an incalculable number of lives.

The Tunny
machine was manufactured by the German Lorenz company.1
The first model bore the designation SZ40, ‘SZ’ standing
for ‘Schlüsselzusatz’ (‘cipher attachment’).
A later version, the SZ42A, was introduced in February 1943, followed
by the SZ42B in June 1944. ‘40’ and ‘42’
appear to refer to years, as in ‘Windows 97’.

The Tunny machine

Tunny was
one of three types of teleprinter cipher machine used by the Germans.
(The North American term for ‘teleprinter’ is ‘teletypewriter’.)
At Bletchley Park (B.P.) these were given the general cover name
‘Fish’. The other members of the Fish family were Sturgeon,
the Siemens and Halske T52
Schlüsselfernschreibmaschine (‘Cipher
Teleprinter Machine’),2 and the unbreakable Thrasher.3
Thrasher was probably the Siemens T43, a one time tape
machine. It was upon Tunny that B.P. chiefly focussed.

The Tunny
machine, which measured 19" by 15½" by 17" high, was
a cipher attachment. Attached to a teleprinter, it
automatically encrypted the outgoing stream of pulses produced by the
teleprinter, or automatically decrypted incoming messages before they
were printed. (Sturgeon, on the other hand, was not an attachment but
a combined teleprinter and cipher machine.) At the sending end of a
Tunny link, the operator typed plain language (the ‘plaintext’ of the
message) at the teleprinter keyboard, and at the receiving end the
plaintext was printed out automatically by another teleprinter
(usually onto paper strip, resembling a telegram). The transmitted
‘ciphertext’ (the encrypted form of the message) was not seen by the
German operators. With the machine in ‘auto’ mode, many
long messages could be sent one after another—the plaintext was
fed into the teleprinter equipment on pre-punched paper tape and was
encrypted and broadcast at high speed. Enigma was clumsy by
comparison. A cipher clerk typed the plaintext at the keyboard of an
Enigma machine while an assistant painstakingly noted down the
letters of the ciphertext as they appeared one by one at the
machine’s lamp-board. A radio operator then transmitted the
ciphertext in the form of Morse code. Morse code was not used with
Tunny: the output of the Tunny machine, encrypted teleprinter code,
went directly to air.4

International teleprinter code assigns a pattern of five pulses and
pauses to each character. Using the Bletchley convention of
representing a pulse by a cross and no pulse by a dot, the letter C,
for example, is •xxx•:
no-pulse, pulse, pulse, pulse, no-pulse. More examples: O is •••xx,
L is •x••x,
U is xxx••,
and S is x•x••.
(The complete teleprinter alphabet is shown in Appendix 1: The teleprinter alphabet.) When a
message in teleprinter code is placed on paper tape, each letter (or
other keyboard character) takes the form of a pattern of holes
punched across the width of the tape. A hole corresponds to a pulse
(cross).

The first Tunny radio link, between Berlin and
Athens/Salonika, went into operation on an experimental basis in June
1941.5
In October 1942 this experimental link closed down, and for a short
time it was thought that the Germans had abandoned the Tunny
machine.6
Later that same month Tunny reappeared in an altered form, on a link
between Berlin and Salonika and on a new link between Königsberg
and South Russia.7
At the time of the allied invasion in 1944, when the Tunny system had
reached its most stable and widespread state,8
there were 26 different links known to the British.9
B.P. gave each link a piscine name: Berlin-Paris was Jellyfish,
Berlin-Rome was Bream, Berlin-Copenhagen Turbot (see right-hand column). The two central
exchanges for Tunny traffic were Strausberg near Berlin for the
Western links, and Königsberg for the Eastern links into
Russia.10
In July 1944, the Königsberg exchange closed and a new hub was
established for the Eastern links at Golssen, about 20 miles from the
Wehrmacht’s underground command headquarters south of Berlin.
During the final stages of the war, the Tunny network became
increasingly disorganised.11
By the time of the German surrender, the central exchange had been
transported from Berlin to Salzburg in Austria.12

There were also fixed exchanges at some other large centres, such as
Paris.13
Otherwise, the distant ends of the links were mobile. Each mobile
Tunny unit consisted of two trucks.14
One carried the radio equipment, which had to be kept well away from
teleprinters for fear of interference. The other carried the
teleprinter equipment and two Tunny machines, one for sending and one
for receiving. This truck also carried a device for punching tapes
for auto transmission. Sometimes a land line was used in preference
to radio.15
In this case, the truck carrying the Tunnies was connected up
directly to the telephone system. (Only Tunny traffic sent by radio
was intercepted by the British.)

As with the Enigma, the heart of the Tunny machine was a system of
wheels (see right-hand column). Some or all of the wheels moved each time the operator typed
a character at the teleprinter keyboard (or in the case of an ‘auto’
transmission from a pre-punched tape, each time a new letter was read
in from the tape). There were twelve wheels in all. They stood side
by side in a single row, like plates in a dish rack. As in the case
of Enigma, the rim of each wheel was marked with numbers, visible to
the operator through a window, and somewhat like the numbers on the
rotating parts of a combination lock.

From October 1942 the operating procedure was this. Before starting
to send a message, the operator would use his thumb to turn the
wheels to a combination that he looked up in a codebook containing
one hundred or more combinations (known as the QEP book). At B.P.
this combination was called the setting for that particular
message. The wheels were supposed to be turned to a new setting at
the start of each new message (although because of operator error
this did not always occur). The operator at the receiving end, who
had the same QEP book, set the wheels of his Tunny machine to the
same combination, enabling his machine to decrypt the message
automatically as it was received. Once all the combinations in a QEP
book had been used it was replaced by a new one.

M

+

N

=

T

•

+

•

=

•

•

+

•

=

•

x

+

x

=

•

x

+

x

=

•

x

+

•

=

x

Adding the letter N to the letter M produces T

T

+

N

=

M

•

+

•

=

•

•

+

•

=

•

•

+

x

=

x

•

+

x

=

x

x

+

•

=

x

Adding N to T produces M

The Tunny machine encrypted each letter of the message by adding
another letter to it. (The process of adding letters together is
explained in the next paragraph.) The internal mechanism of the Tunny
machine produced its own stream of letters, known at B.P. as the
‘key-stream’, or simply key. Each letter of the
ciphertext was produced by adding a letter from the key-stream to the
corresponding letter of the plaintext.

The Tunny machine adds letters by adding the individual dots and
crosses that compose them. The rules that the makers of the machine
selected for dot-and-cross addition are simple. Dot plus dot is dot.
Cross plus cross is dot. Dot plus cross is cross. Cross plus dot is
cross. In short, adding two sames produces dot, and adding a mixed
pair produces cross. (Computer literati will recognise Tunny addition
as boolean XOR.)

For example, if the first letter of the plaintext happens to be M,
and the first letter of the key-stream happens to be N, then the
first letter of the ciphertext is T: adding M (••xxx)
and N (••xx•)
produces T (••••x).

The German engineers selected these rules for dot-and-cross addition
so that the following is always true (no matter which letters, or
other keyboard characters, are involved): adding one letter (or other
character) to another and then adding it again a second time
leaves you where you started. In symbols, (x + y) + x
= y, for every pair of keyboard characters x and y.
For example, adding N to M produces T, as we have just seen, and then
adding N to T leads back to M (see right-hand column).

This explains how the receiver’s Tunny decrypted the
ciphertext. The ciphertext was produced by adding a stream of key to
the plaintext, so by means of adding exactly the same letters of key
to the ciphertext, the receiver’s machine wiped away the
encryption, exposing the plaintext again.

For example, suppose the plaintext is the single word ‘COLOSSUS’.
The stream of key added to the plaintext by the sender’s Tunny
might be: WZHI/NR9. These characters are added serially to the
letters of ‘COLOSSUS’:

C+W O+Z L+H O+I S+/ S+N U+R S+9.

This
produces

XDIVSDFE

(as can be
checked by using the table in Appendix 1). ‘XDIVSDFE’ is
transmitted over the link. The Tunny at the receiving end adds the
same letters of key to the encrypted message:

Tunny. Wheels 1–5 are the psi-wheels, wheels 6 and 7 are the motor-wheels, and wheels 8–12 are the chi-wheels.11

The
Tunny machine in fact produces the key-stream by adding together two
other letter streams, called at B.P. the psi-stream and the
chi-stream (from the Greek letters psi (ψ)
and chi (χ)). The psi-stream and
the chi-stream are produced by the wheels of the Tunny machine. Let
us consider the wheels in more detail.

The twelve wheels form three groups: five psi-wheels, five
chi-wheels, and two motor wheels. Each wheel has different numbers of
cams (sometimes called ‘pins’) arranged evenly around its
circumference (the numbers varying from 23 to 61). The function of
the cam is to push a switch as it passes it, so that as the wheel
rotates a stream of electrical pulses is generated. The operator can
adjust the cams, sliding any that he selects sideways, so that they
become inoperative and no longer push the switch when they pass it (see right-hand column).
The wheel now causes not a uniform stream of pulses as it turns, but
a pattern of pulses and non-pulses—crosses and dots. The
arrangement of the cams around the wheel, operative or inoperative,
is called the wheel pattern.

Prior to the summer of 1944 the Germans changed the cam patterns of
the chi-wheels once every month and the cam patterns of the
psi-wheels at first quarterly, then monthly from October 1942. After
1 August 1944, wheel patterns changed daily. The changes were made
according to books of wheel patterns issued to Tunny units (different
links used different books).

It is the patterns of the cams around the wheels that produces the
chi-stream and the psi-stream. Whenever a key is pressed at the
keyboard (or a letter read in from the tape in ‘auto’
mode), it causes the five chi-wheels to turn in unison, just far
enough for one cam on each wheel to pass its switch. Depending on
whether or not that cam is operative, a pulse may or may not be
produced. Suppose, for example, that the cam at the first chi-wheel’s
switch produces no pulse and the cam on the second likewise produces
no pulse at its switch, but the cams on the third and fourth both
produce a pulse, and the cam on the fifth produces no pulse. Then the
pattern that the chi-wheels produce at this point in their rotation
is ••xx•.
In other words, the chi-stream at this point contains the letter N.
The five psi-wheels also contribute a letter (or other keyboard
character) and this is added to N to produce a character of the
key-stream.

A complication in the motion of the wheels is that, although the
chi-wheels move forward by one cam every time a key is pressed
at the keyboard (or a letter arrives from the tape in auto mode, or
from the radio receiver), the psi-wheels move irregularly. The psis
might all move forward with the chis, or they might all stand still,
missing an opportunity to move. This irregular motion of the
psi-wheels was described as ‘staggering’ at B.P. Designed to enhance
the security of the machine, it turned out to be the crucial
weakness.

Whether the psi-wheels move or not is determined by the motor wheels
(or in some versions of the machine, by the motor wheels in
conjunction with yet other complicating factors). While the psis
remain stationary, they continue to contribute the same letter to the
key. So the chis might contribute

...KDUGRYMC...

and the
psis might contribute

...GGZZZWDD...

Here the
chis have moved eight times and the psis only four.

A sample decrypt

To OKH/OP. ABT. and to OKH/Foreign Armies East,
from Army Group South IA/01, No. 411/43, signed von Weichs, General
Feldmarschall, dated 25/4:-

Comprehensive appreciation of the enemy for
"Zitadelle"

In
the main the appreciation of the enemy remains the same as reported
in Army Group South (Roman) IIA, No. 0477/43 of 29/3 and in the
supplementary appreciation of 15/4. [In Tunny transmissions the
word ‘Roman’ was used to indicate a Roman numeral; ‘29/3’ and ‘15/4’
are dates.]

The
main concentration, which was already then apparent on the north
flank of the Army Group in the general area
Kursk--Ssudsha--Volchansk--Ostrogoshsk,
can now be clearly recognised: a further intensification of this
concentration is to be expected as a result of the continuous heavy
transport movements on the lines Yelets--Kastornoye--Kursk,
and Povorino--Svoboda
and Gryazi--Svoboda,
with a probable (B% increase) [‘B%’ indicated an uncertain word]
in the area Valuiki--Novy
Oskol--Kupyansk.
At present however it is not apparent whether the object of this
concentration is offensive or defensive. At present, (B% still) in
anticipation of a German offensive on both the Kursk and MiusDonetz fronts, the armoured and mobile
formations are still evenly distributed in various groups behind the
front as strategic reserves.

There
are no signs as yet of a merging of these formations or a transfer to
the forward area (except for (Roman) II GDS [Guards] Armoured
Corps) but this could take place rapidly at any time.

According
to information from a sure source the existence of the following
groups of the strategic reserve can be presumed:-
A) 2 cavalry corps (III
GDS and V GDS in
the area north of Novocherkassk). It can also be presumed that 1 mech
[mechanised] corps (V
GDS) is being brought up to strength here. B) 1 mech corps (III
GDS) in the area (B% north) of Rowenki. C) 1 armoured corps, 1
cavalry corps and probably 2 mech corps ((Roman) I GD Armoured, IV
Cavalry, probably (B% (Roman) I) GDS Mech and V Mech Corps) in the
area north of Voroshilovgrad. D) 2 cavalry corps ((B% IV) GDS and VII
GDS) in the area west of Starobyelsk. E) 1 mech corps, 1 cavalry
corps and 2 armoured corps ((Roman) I GDS (B% Mech), (Roman) I GDS
Cavalry, (Roman) II and XXIII Armoured) in
the area of Kupyansk--Svatovo.
F) 3 armoured corps, 1 mech corps ((Roman) II Armoured, V GDS
Armoured, (B% XXIX) Armoured and V GDS Mech under the command of an
army (perhaps 5 Armoured Army)) in
the area of Ostrogoshsk. G) 2 armoured and 1 cavalry corps ((Roman)
II GDS Armoured, III GDS Armoured and VI GDS Cavalry) under the
command of an unidentified H.Q., in the area north of Novy Oskol.

In
the event of "Zitadelle", there are at present
approximately 90 enemy formations west of the line
Belgorod--Kursk--Maloarkhangelsk.
The attack of the Army Group will encounter stubborn enemy resistance
in a deeply echeloned and well developed
main defence zone, (with numerous dug in
tanks, strong artillery and local reserves) the main effort of the
defence being in the key sector Belgorod--Tamarovka.

In
addition strong counter attacks
by strategic reserves from east and southeast are to be expected. It
is impossible to forecast whether the enemy will attempt to withdraw
from a threatened encirclement by retiring eastwards, as soon as the
key sectors [literally, ‘corner-pillars’] of the bulge in the
frontline at Kursk, Belgorod and Maloarkhangelsk, have been broken
through. If the enemy throws in all strategic reserves on the Army
Group front into the Kursk battle, the following may appear on the
battle field:- On day 1 and day 2, 2 armoured divisions and 1 cavalry
corps. On day 3, 2 mech and 4 armoured corps. On day 4, 1 armoured and 1 cavalry corps. On day 5,
3 mech corps. On day 6, 3 cavalry corps. On day 6 and/or day 7,
2 cavalry corps.

Summarizing,
it can be stated that the balance of evidence still points to a
defensive attitude on the part of the enemy: and this is in fact
unmistakable in the frontal sectors of the 6 Army and 1 Panzer Army.
If the bringing up of further forces in the area before the north
wing of the Army Group persists and if a transfer forward and merging
of the mobile and armoured formations then takes place, offensive
intentions become more probable. In that case it is improbable that
the enemy can even then forestall our execution of Zitadelle in the
required conditions. Probably on the other hand we must assume
complete enemy preparations for defence, including the counter
attacks of his strong mot [motorised]
and armoured forces, which must be expected.14

The right-hand column contains
is a rare survivor—a word-for-word translation of an
intercepted Tunny message.15
Dated 25 April 1943 and signed by von Weichs, Commander-in-Chief of
German Army Group South, this message was sent from the Russian front
to the German Army High Command (‘OKH’—Oberkommando des
Heeres). It gives an idea of the nature and quality of the
intelligence that Tunny yielded. The enciphered message was
intercepted during transmission on the ‘Squid’ radio
link
between the headquarters of Army Group South and Königsberg.17

The message concerns plans for a major German
offensive in the Kursk area codenamed ‘Zitadelle’. Operation
Zitadelle was Hitler’s attempt to regain the initiative on the
Eastern Front following the Russian victory at Stalingrad in February
1943. Zitadelle would turn out to be one of the crucial
battles of the war. Von Weichs’ message gives a detailed appreciation
of Russian strengths and weaknesses in the Kursk area. His
appreciation reveals a considerable amount about the intentions of
the German Army. British analysts deduced from the decrypt that
Zitadelle would consist of a pincer attack on the north and
south flanks (‘corner-pillars’) of a bulge in the Russian defensive
line at Kursk (a line which stretched from the Gulf of Finland in the
north to the Black Sea in the south).18
The attacking German forces would then attempt to encircle the
Russian troops situated within the bulge.

Highly important messages such as this were
conveyed directly to Churchill, usually with a covering note by ‘C’,
Chief of the Secret Intelligence Service.19
On 30 April an intelligence report based on the content of the
message, but revealing nothing about its origin, was sent to
Churchill’s ally, Stalin.20
(Ironically, however, Stalin had a spy inside Bletchley Park: John
Cairncross was sending raw Tunny decrypts directly to Moscow by
clandestine means.21)

The Germans finally launched operation Zitadelle on 4 July
1943.22 Naturally the German offensive came as no
surprise to the Russians—who, with over two months warning of
the pincer attack, had amassed formidable defences. The Germans threw
practically every panzer division on the Russian front into Zitadelle,23 but to no avail, and on 13 July Hitler called
off the attack.24 A few days later Stalin announced
in public that Hitler’s plan for a summer offensive against the
Soviet Union had been ‘completely frustrated’.25Zitadelle—the Battle of
Kursk—was a decisive turning point on the Eastern front. The
counter attack launched by the Russians during Zitadelle
developed into an advance which moved steadily westwards, ultimately
reaching Berlin in April 1945.

Colossus
was the brainchild of Thomas H. Flowers (1905–1998). Flowers joined
the Telephone Branch of the Post Office in 1926, after an
apprenticeship at the Royal Arsenal in Woolwich (well-known for its
precision engineering). Flowers entered the Research Branch of the
Post Office at Dollis Hill in North London in 1930, achieving rapid
promotion and establishing his reputation as a brilliant and
innovative engineer. At Dollis Hill Flowers pioneered the use of
large-scale electronics, designing equipment containing more than
3000 electronic valves (‘vacuum tubes’ in the US). First summoned to
Bletchley Park to assist Turing in the attack on Enigma, Flowers soon
became involved in Tunny. After the war Flowers pursued his dream of
an all-electronic telephone exchange, and was closely involved with
the groundbreaking Highgate Wood exchange in London (the first
all-electronic exchange in Europe).

Max Newman. Head of the Tunny-breaking section called the ‘Newmanry’, Newman was in charge of the Colossi. He went on to found the Computing Machine Laboratory at Manchester University.16

Colonel John Tiltman (right), with Alastair Denniston, Head of the Government Code and Cypher School from 1919 (left), and ‘Vinca’ Vincent, an expert on Italian ciphers. Tiltman achieved the first break into Tunny.17

Alan M. Turing. Turing made numerous fundamental contributions to code-breaking, and he is the originator of the modern (‘stored-program’) computer.18

Max H. A.
Newman (1897–1984) was a leading topologist as well as a pioneer of
electronic digital computing. A Fellow of St John’s College,
Cambridge, from 1923, Newman lectured Turing on mathematical logic in
1935, launching Turing26on the research that ledto the ‘universal Turing
machine’, the abstract universal stored-program computer described in
Turing’s 1936 paper ‘On Computable Numbers’. At the end of August 1942
Newman left Cambridge for Bletchley Park, joining the Research
Section and entering the fight against Tunny. In 1943 Newman became
head of a new Tunny-breaking section known simply as the Newmanry,
home first to the experimental ‘Heath Robinson’ machine
and subsequently to Colossus. By April 1945 there were ten Colossi
working round the clock in the Newmanry. The war over, Newman took up
the Fielden Chair of Mathematics at the University ofManchester
and—inspired both by Colossus and by Turing’s abstract
‘universal machine’—lost no time in establishing a facility to
build an electronic stored-program computer. On 21 June 1948, in
Newman’s Computing Machine Laboratory, the world’s first
electronic stored-program digital computer, the Manchester ‘Baby’,
ran its first program.

John
Tiltman (1894–1982) was seconded to the Government Code and Cypher
School (GC & CS) from the British army in 1920, in order to
assist with Russian diplomatic traffic.27 An instant success as a codebreaker, Tiltman never
returned to ordinary army duties. From 1933 onwards he made a series
of major breakthroughs against Japanese military ciphers, and in the
early years of the war he also broke a number of German ciphers,
including the army’s double Playfair system, and the version of
Enigma used by the German railway authorities. In 1941 Tiltman made
the first significant break into Tunny. Promoted to Brigadier in
1944, he went on to become a leading member of GCHQ, GC & CS’s
peacetime successor. Following his retirement from GCHQ in 1964,
Tiltman joined the National Security Agency, where he worked until
1980.

Alan M.
Turing (1912–1954) was elected a Fellow of King’s College,
Cambridge in 1935, at the age of only 22. ‘On Computable
Numbers’, published the following year, was his most important
theoretical work. It is often said that all modern computers are
Turing machines in hardware: in a single article, Turing ushered in
both the modern computer and the mathematical study of the
uncomputable. During the early stages of the war, Turing broke
German Naval Enigma and produced the logical design of the ‘Bombe’,
an electro-mechanical code-breaking machine. Hundreds of Bombes
formed the basis of Bletchley Park’s factory-style attack on
Enigma. Turing briefly joined the attack on Tunny in 1942,
contributing a fundamentally important cryptanalytical method known
simply as ‘Turingery’. In 1945, inspired by his knowledge of
Colossus, Turing designed an electronic stored-program digital
computer, the Automatic Computing Engine (ACE). At Bletchley Park,
and subsequently, Turing pioneered Artificial Intelligence: while the
rest of the post-war world was just waking up to the idea that
electronics was the new way to do binary arithmetic, Turing was
talking very seriously about programming digital computers to think.
He also pioneered the discipline now known as Artificial Life, using
the Ferranti Mark I computer at Manchester University to model
biological growth.28

William
T. Tutte (1917–2002) specialised in chemistry in his undergraduate
work at Trinity College, Cambridge, but was soon attracted to
mathematics. He was recruited to Bletchley Park early in 1941,
joining the Research Section. Tutte worked first on the Hagelin
cipher machine and in October 1941 was introduced to Tunny. Tutte’s
work on Tunny, which included deducing the structure of the Tunny
machine, can be likened in importance to Turing’s earlier work
on Enigma. At the end of the war, Tutte was elected to a Research
Fellowship in mathematics at Trinity; he went on to found the area of
mathematics now called graph theory.

Breaking the Tunny machine

From time
to time German operators used the same wheel settings for two
different messages, a circumstance called a depth. It was
thanks to the interception of depths, in the summer of 1941, that the
Research Section at B.P. first found its way into Tunny.

Prior to October 1942, when QEP books were introduced, the sending
operator informed the receiver of the starting positions of the 12
wheels by transmitting an unenciphered group of 12 letters. The first
letter of the 12 gave the starting position of the first psi-wheel,
and so on for the rest of the wheels. For example, if the first
letter was ‘M’ then the receiver would know from the standing
instructions for the month to set his first psi-wheel to position 31,
say. At B.P. this group of letters was referred to as the message’s
indicator. Sometimes the sending operator would expand the 12
letters of the indicator into 12 unenciphered names: Martha Gustav
Ludwig Otto... instead of MGLO..., for example (see right-hand column). The occurrence of
two messages with the same indicator was the tell-tale sign of a
depth.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

J

S

H

5

N

Z

Y

M

F

S

0

1

1

5

I

V

K

U

1

Y

U

4

N

C

E

J

E

G

P

B

J

S

H

5

N

Z

Y

Z

Y

5

G

L

F

R

G

X

O

5

S

Q

5

D

A

1

J

J

H

D

5

0

0

0

0

0

0

0

0

f

o

u

g

f

1

4

m

a

q

s

g

5

s

e

k

z

r

0

y

w

h

e

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

M

N

T

Q

M

A

0

U

4

Y

L

1

Q

I

J

L

Y

V

I

N

U

B

2

3

R

5

W

E

V

G

B

K

S

U

C

B

T

T

O

5

E

4

T

S

L

E

3

F

G

Z

Y

U

H

V

H

3

H

E

E

0

s

a

y

t

l

g

t

q

t

q

w

q

u

a

b

w

c

w

m

x

l

v

t

s

v

b

u

0

1

g

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

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The first 120 characters of the two transmissions attacked by Tiltman. The letters shown in green are the result of ‘cancelling out’ the key by adding the two transmissions together.21

So when on 30 August 1941 two messages with the same indicator were
intercepted, B.P. suspected that they had found a depth. As it turned
out, the first transmission had been corrupted by atmospheric noise,
and the message was resent at the request of the receiving operator.
Had the sender repeated the message identically, the use of the same
wheel settings would have left B.P. none the wiser. However, in the
course of the second transmission the sender introduced abbreviations
and other minor deviations (the message was approximately 4000
characters long). So the depth consisted of two not-quite-identical
plaintexts each encrypted by means of exactly the same sequence of
key—a codebreaker’s dream.

On the hypothesis that the machine had produced the ciphertext by
adding a stream of key to the plaintext, Tiltman added the two
ciphertexts (see right-hand column). If the hypothesis were correct, this would have the
effect of cancelling out the key (since, as previously mentioned, ((x
+ y) + x) = y). The resulting string of
approximately 4000 characters would consist of the two plaintexts
summed together character by character. (This is because (K +
P) + (K + P) = ((K + P) + K)
+ P = P + P, where K is the key, P
is the plaintext, and K + P is the ciphertext.)

Tutte deduced the design of the Tunny machine from the pair of intercepts shown above.22

The stricken U-110 shortly after depth charges blasted it to the surface. A Royal Navy boarding party captured the U-110’s Enigma machine.24

Tiltman managed to prise the two individual plaintexts out of this
string (it took him ten days). He had to guess at words of each
message, and Tiltman was a very good guesser. Each time he guessed a
word from one message, he added it to the characters at the right
place in the string, and if the guess was correct an intelligible
fragment of the second message would pop out. For example, adding the
probable word ‘geheim’ (secret) to characters 83-88 of the string
reveals the plausible fragment ‘eratta’.29 This short
break can then be extended to the left and right. More letters of the
second message are obtained by guessing that ‘eratta’ is part of
‘militaerattache’ (military attache), and if these letters are added
to their counterparts in the string, further letters of the first
message are revealed. And so on. Eventually Tiltman achieved enough
of these local breaks to realise that long stretches of each message
were the same, and so was able to decrypt the whole thing.

Adding
the plaintext deduced by Tiltman to its corresponding ciphertext
revealed the sequence of key used to encrypt the messages. These 4000
characters of key were passed to Tutte and, in January 1942, Tutte
single-handedly deduced the fundamental structure of the Tunny
machine. He focussed on just one of the five ‘slices’ of
the key-stream, the top-most row were the key-stream to be punched on
tape. Each of these five slices was called an ‘impulse’
at B.P. (In the ‘Colossus’ punched tape shown earlier,
the first impulse is ••••xxxx,
the second is x•x•••x•,
and so on.)

The
top-most impulse of the key-stream, Tutte managed to deduce, was the
result of adding two streams of dots and crosses. The two streams
were produced by a pair of wheels, which he called ‘chi’
and ‘psi’. The chi-wheel, he determined, always moved
forward one place from one letter of text to the next, and the
psi-wheel sometimes moved forwards and sometimes stayed still. It was
a remarkable feat of cryptanalysis. At this stage the rest of the
Research Section joined in and soon the whole machine was laid bare,
without any of them ever having set eyes on one.

Turingery

Now that Bletchley knew
the nature of the machine, the next step was to devise methods for
breaking the daily traffic. A message could be read if the wheel
settings and the wheel patterns were known. The German operators
themselves were revealing each message’s setting via the 12-letter
indicator. Thanks to Tutte’s feat of reverse-engineering, the
wheel patterns were known for August 1941. The codebreaker’s problem
was to keep on top of the German’s regular changes of wheel-pattern.

In
July 1942 Turing invented a method for finding wheel-patterns from
depths—‘Turingery’. Turing was at that time on loan
to the Research Section from Hut 8 and the struggle against Naval
Enigma.30 Turingery was the third of the three
strokes of genius that Turing contributed to the attack on the German
codes, along with his design for the Bombe and his unravelling of the
form of Enigma used by the Atlantic U-boats.31 As
fellow codebreaker Jack Good observed, ‘I won’t say that
what Turing did made us win the war, but I daresay we might have lost
it without him’.32

Turingery was a hand method, involving paper, pencil and eraser.
Beginning with a stretch of keyobtained from a depth,
Turingery enabled the breaker to prize out from the key the
contribution that the chi-wheels had made. The cam-patterns of the
individual chi-wheels could be inferred from this. Further deductions
led to the cam-patterns of the psi- and motor-wheels. Once gained via
Turingery, this information remained current over the course of many
messages. Eventually the patterns were changed too frequently for any
hand method to be able to cope (there were daily changes of all
patterns from August 1944), but by that time Colossus, not Turingery,
was being used for breaking the wheel patterns.

Basic to Turingery was the idea of forming the delta of a
stream of characters. (Delta-ing a character-stream was also called
‘differencing’ the stream.) The delta of a character-stream is the
stream that results from adding together each pair of adjacent
letters in the original stream. For example, the delta of the short
stream MNT (sometimes written ΔMNT)
is produced by adding M to N and N to T (using the rules of
dot-and-cross addition explained previously). The delta of MNT is in fact
TM, as the table in the right-hand column shows (the shaded columns contain the
delta).

‘Delta-ing’ and ‘Turingery’ were Turing’s fundamental contributions to the attack on Tunny.26

The
Essential Turing (Oxford University Press, 2004) gives a comprehensive account of the Bletchley Park codebreaking operation, including Turing’s own description of the Bombe.

The idea of the delta is that it tracks changes in the
original stream. If a dot follows a dot or a cross follows a cross at
a particular point in the original stream, then the corresponding
point in the delta has a dot (see the table). A dot in the delta
means ‘no change’. When, on the other hand, there is a
cross followed by a dot or a dot followed by a cross in the original
stream, then the corresponding point in the delta has a cross. A
cross in the delta means ‘change’. Turing introduced the
concept of delta in July 1942, observing that by delta-ing a stretch
of key he was able to make deductions which could not be made from
the key in its un-deltaed form.33

Turingery worked on deltaed key to produce the deltaed contribution
of the chi-wheels. Turing’s discovery that delta-ing would reveal
information otherwise hidden was essential to the developments that
followed. The algorithms implemented in Colossus (and in its
precursor Heath Robinson) depended on this simple but brilliant
observation. In that sense, the entire machine-based attack on Tunny
flowed from this fundamental insight of Turing’s.

How did Turingery work? The method exploited the fact that each
impulse of the chi-stream (and also its delta-ed form) consists of a
pattern that repeats after a fixed number of steps. Since the number
of cams on the 1st chi-wheel is 41, the pattern in the first impulse
of the chi-stream repeats every 41 steps. In the 2nd impulse the
pattern repeats every 31 steps—the number of cams on the 2nd
chi-wheel—and for the 3rd, 4th and 5th impulses, the wheels
have 29, 26, and 23 cams respectively. Therefore a hypothesis about
the identity, dot or cross, of a particular bit in, say, the first
impulse of the chi will, if correct, also produce the correct bit 41
steps further on, and another 41 steps beyond that, and so on. Given
500 letters of key, a hypothesis about the identity of a single
letter of the chi (or delta-ed chi) will yield approximately 500/41
bits of the first impulse, 500/31 bits of the second impulse, 500/29
bits of the third, and so on—a total of about 85 bits.

In outline, Turing’s method is this. The first step is to make a
guess: the breaker guesses a point in the delta-ed key at which the
psi-wheels stayed still in the course of their ‘staggering’ motion.
Whatever guess is made, it has a 50% chance of being right. Positions
where the psis did not move are of great interest to the breaker,
since at these positions the deltaed key and the deltaed chi are
identical. (The reason for this is that the deltaed contribution of
the psis at such positions is •••••, and
adding ••••• to a letter does not alter the
letter.) Because the key is known, the letter of the deltaed chi at
the guessed position is also known—assuming, of course, that
the guess about the psis not having moved is correct. Given this
single letter of the deltaed chi, a number of bits can then be filled
in throughout the five impulses, by propagating to the left and right
at the appropriate periods.

Now that various bits of the delta chi are filled in, guesses can be
made as to the identity of others letters. For example, if one letter
of the delta chi is •???• and the corresponding letter of
the delta key is •xxx•
(C), the breaker may guess that this is another point at which the
psis stood still, and replace •???• in the delta chi by
•xxx•. This
gives three new bits to propagate left and right. And so the process
continues, with more and more bits of the delta chi being written in.

Naturally the breaker’s guesses are not always correct, and as the
process of filling in bits goes on, any incorrect guesses will tend
to produce clashes—places where both a cross and a dot are
assigned to the same position in the impulse. Guesses that are
swamped by clashes have to be revised. With patience, luck, a lot of
rubbing out, and a lot of cycling back and forth between putative
fragments of delta chi and delta psi, a correct and complete stretch
of delta chi eventually emerges.

Tutte’s statistical method

Ralph Tester – head of the Tunny-breaking section called the ‘Testery’.27

Tunny
could now be tackled operationally, and a Tunny-breaking section was
immediately set up under Major Ralph Tester.34
Several members of the Research Section moved over to the ‘Testery’.
Armed with Turingery and other hand methods, the Testery read nearly
every message from July to October 1942—thanks
to the insecure 12-letter indicator system, by means of which the
German operator obligingly conveyed the wheel setting to the
codebreakers.35 In October, however, the indicators were replaced by
numbers from the QEP books, and the Testery, now completely reliant
on depths, fell on leaner times. With the tightening up of German
security, depths were becoming increasingly scarce. The Research
Section renewed its efforts against Tunny, looking for a means of
finding wheel settings that did not depend on depths.36

In
November 1942 Tutte invented a way of discovering the settings of
messages not in depth. This became known as the ‘Statistical
Method’. The rub was that at first Tutte’s method seemed
impractical. It involved calculations which, if done by hand, would
consume a vast amount of time—probably as much as several
hundred years for a single, long message, Newman once estimated.37

The necessary calculations were straightforward enough, consisting
basically of comparing two streams made up of dots and crosses, and
counting the number of times that each had a dot, or cross, in the
same position. Today, of course, we turn such work over to electronic
computers. When Tutte shyly explained his method to Newman, Newman
suggested using high-speed electronic counters to mechanise the
process. It was a brilliant idea. Within a surprisingly short time a
factory of monstrous electronic computers dedicated to breaking Tunny
was affording a glimpse of the future.

Electronic counters had been developed in Cambridge before the war.
Used for counting emissions of sub-atomic particles, these had been
designed by C. E. Wynn-Williams, a Cambridge don.38 Newman knew
of Wynn-Williams’ work, and in a moment of inspiration he saw
that the same idea could be applied to the Tunny problem. Within a
month of Tutte’s inventing his statistical method Newman began
developing the necessary machine. He worked out the cryptanalytical
requirements for the machine and called in Wynn-Williams to design
the electronic counters. Construction of Newman’s machine
started in January 1943 and a prototype began operating in June of
that year, in the newly formed Tunny-breaking section called the
‘Newmanry’. The prototype machine was soon dubbed ‘Heath Robinson’,
after the famous cartoonist who drew overly-ingenious mechanical
contrivances.

Tutte’s method delivered the settings of the chi wheels. Once the
Newmanry had discovered the settings of the chis by machine, the
contribution that the chis had made to the ciphertext was stripped
away, producing what was called the ‘de-chi’ of the
message. The de-chi was made by a replica of the Tunny machine,
designed by Flowers’ Post Office engineers at Dollis Hill. The de-chi
was then passed to the Testery, where a cryptanalyst would break into
it by ‘ordinary’ pencil-and-paper methods requiring only (as a
wartime document described it) ‘the power of instantaneous
mental addition of letters of the Teleprint alphabet’.39

The reason it was feasible to break the de-chi by hand was that the
staggering motion of the psi-wheels introduced local regularities.
Once the contribution of the chis had been stripped out of the key,
what remained of the key contained distinctive patterns of repeated
letters, e.g. ...GGZZZWDD..., since while the psis stood still they
continued to contribute the same letter. By latching onto these
repetitions, the cryptanalyst could uncover some stretches of this
residual key, and this in turn enabled the settings of the psi-wheels
and the motor-wheels to be deduced. For example, adding the guessed
word ‘dringend’ (‘urgent’) to the de-chi near the beginning of the
message might produce 888EE00WW—pure gold, confirming the
guess. With luck, once a break was achieved it could be extended to
the left or right, in this case perhaps by trying on the left ‘sehr9’
(‘very’ followed by a space), and on the right ++M88, the code for a
full stop (see Appendix 1). Once the codebreaker had a short stretch
of the key that the psi-wheels had contributed, the wheel settings
could usually be obtained by comparing the key to the known wheel
patterns. When all the wheel-settings were known, the ciphertext was
keyed into one of the Testery’s replica Tunny machines, and the
German plaintext would emerge.

In order to illustrate the basic ideas of Tutte’s method for
finding the settings of the chi wheels, let us assume that we have an
intercepted ciphertext 10,000 characters long. This ciphertext is
punched on a tape (we call this the ‘message-tape’). An
assistant, who knows the chi-wheel patterns, provides us with a
second tape (the ‘chi-tape’). This assistant has worked
out the machine’s entire chi-stream, beginning at an
arbitrarily selected point in the revolution of the chi-wheels, and
stepping through all their possible joint combinations. (Once the
wheels have moved through all the possible combinations, their
capacity for novelty is exhausted, and should the wheels continue to
turn they merely duplicate what has gone before.) The complete
chi-stream is, of course, rather long, but eventually the assistant
does produce a roll of tape with the stream punched on it. The
sequence of 10,000 consecutive characters of chi-stream that was used
to encrypt our message is on this tape somewhere—our problem is
to find it. This sequence is called simply ‘the chi’ of the message.
Tutte’s method exploits a fatal weakness in the design of the
Tunny machine, a weakness again stemming from the staggering motion
of the psi-wheels. The central idea of the method is this: The chi
is recognisable on the basis of the ciphertext, provided the wheel
patterns are known. Tutte showed by a clever mathematical
deduction that the delta of the ciphertext and the delta of the chi
would usually correspond slightly. That slightly is the key to
the whole business—any degree of regularity, no matter how
weak, is the cryptanalyst’s friend. The slight regularity that
Tutte discovered could be used as a touchstone for finding the chi.
(Readers interested in Tutte’s mathematical reasoning will find the
details in Appendix 2: The
Tunny encipherment equation and Tutte’s 1 + 2 break-in. At present we will concentrate on how the
method is carried out.)

We select the first 10,000 characters of the chi-tape; we will
compare this stretch of the chi-tape with the message-tape. Tutte
showed that in fact we need examine only the first and the
second of the five horizontal rows punched along the chi-tape,
the first and second impulses (these two rows are the contributions
of the first and second chi-wheels respectively). Accordingly we need
consider only the first and second impulses of the message-tape. This
simplifies considerably the task of comparing the two tapes. Because
Tutte’s method focussed on the first and second chi-wheels it
was dubbed the ‘1+2 break in’.40

Here is the procedure for comparing the message-tape with the stretch
of chi-tape we have picked. First, we add the first and second
impulses of the message-tape and form the delta of the resulting
sequence of dots and crosses. (For example, if the sequence produced
by adding the two impulses begins x•x...,
the delta begins xx... .) Second, we add the first and second impulses of the
10,000-character piece of chi-tape, and again form the delta of the
result. Next we lay these two deltas side by side and count how many
times they have dots in the same places and how many times crosses.
We add the two tallies to produce a total score for this particular
piece of the chi-tape. We are looking for a match between the two
deltas of around 55%. Tutte showed that this is the order of
correspondence that can be expected when the piece of chi-tape under
examination contains the first and second impulses of the actual chi.

Stepping the ciphertext through the chi-stream, looking for
the starting position of the chi-wheels.33

The first score we obtain probably won’t be anything
special—for we would be extremely lucky if the first 10,000
characters of chi-stream that we examined were the chi of the
message. So next we shift along one character in the chi-stream and
focus on a new candidate for the message’s chi, the 2nd through
to the 10,001st characters on the chi-tape (see the diagram in the right-hand column). We add,
delta, and count once again. Then we shift along another character,
repeating the process until we have examined all candidates for the
chi. A buoyant score reveals the first and second impulses of the
actual chi (we hope).

Once a winning segment of the chi-tape has been located, its place
within the complete chi-stream tells us the positions of the first
and second chi-wheels at the start of the message. With these
settings in hand, a similar procedure is used to chase the settings
of the other chi-wheels.

As mentioned previously, the cause of the slight regularity that
Tutte latched onto is at bottom the staggering movement of the
psi-wheels—the great weakness of the Tunny machine. While the
psis remained stationary, they continued to contribute the same
letter to the key; and so, since delta-ing tracks change, the delta
of the stream of characters contributed by the psis contained more
dots than crosses (recall that a cross in the delta indicates a
change). Tutte calculated that there would usually be about 70% dot
in the delta of the sum of the contributions of the first two
psi-wheels.

The delta of the plaintext also contained more dots than crosses (for
reasons explained in Appendix 2, which included the fact that Tunny
operators habitually repeated certain characters). Tutte investigated
a number of broken messages and discovered to his delight that the
delta of the sum of the first two impulses was as a rule about 60%
dot. Since these statistical regularities in the delta of the psi and
the delta of the plain both involved a predominance of dot over
cross, they tended to reinforce one another. Tutte deduced that their
net effect, in favourable cases, would be the agreement, noted above,
of about 55% between the processed ciphertext and the processed chi.

This machine, eventually called ‘Old Robinson’, replaced the original Heath Robinson (the two were of similar
appearance). To the left are the two large metal frames called
‘bedsteads’, which held the tape-drive mechanism, the photo-electric
readers, and the two tapes supported by pulleys. One tape contained
the ciphertext and the other held impulses from the chi-wheels of the
Tunny machine. To the right are the ‘combining unit’ and the
electronic counters.35

Colossus. In the foreground is the automatic typewriter for
output. The large frames to the right held two message tapes. As one
job was being run, the tape for the next job would be loaded onto the
pulleys, so saving time. Using a switch on the selection panel, the
operator chose to run either the ‘near’ or the ‘far’ tape.43

Side view of Colossus VII. The four large boxes on the rear frame are the power supply units.44Colossus, from a sketch by Flowers.45

Tunny’s security depended on the appearance of randomness, and
here was a crack in the appearance. The British seized on it. If,
instead of the psi-wheels either all moving together or all standing
still, the designers had arranged for them to move independently—or
even to move regularly like the chis—then the chink that let
Tutte in would not have existed.

Heath Robinson

Smoke rose
from Newman’s prototype machine the first time it was switched on (a
large resistor overloaded). Around a vast frame made of angle-iron
wound two long loops of teleprinter tape (see photo). Resembling an old-fashioned
bed standing on end, the frame quickly became known as the
‘bedstead’. The tapes were supported by a system of
pulleys and wooden wheels of diameter about ten inches. Each tape was
driven by a toothed sprocket-wheel which engaged a continuous row of
sprocket-holes along the centre of the tape (see previous diagram). The tapes
were driven by the same drive-shaft and moved in synchronisation with
each other at a maximum speed of 2000 characters per second. To the
amusement and annoyance of Heath Robinson’s operators, tapes
would sometimes tear or come unglued, flying off the bedstead at high
speed and breaking into fragments which festooned the Newmanry.

One tape was the message-tape and the other the chi-tape. In practice
the chi-tape might contain, for example, only the first and second
impulses of the complete chi-stream, resulting in a shorter tape. The
drive mechanism was arranged so that as the tapes ran on the
bedstead, the message-tape stepped through the chi-tape one character
at a time (see previous diagram). Photo-electric readers mounted on the
bedstead converted the hole/no-hole patterns punched on the tapes
into streams of electrical pulses, and these were routed to a
‘combining unit’—a logic unit, in modern
terminology. The combining unit did the adding and the delta-ing, and
Wynn-Williams’ electronic counters produced the scores. The way
the combining was done could be varied by means of replugging cables,
a primitive form of programming. The combining unit, the bedstead
and the photo-electric readers were made by Post Office engineers at
Dollis Hill and the counters by Wynn-Williams’ unit at the
Telecommunications Research Establishment (TRE).41

Heath Robinson worked, proving in a single stroke that Newman’s
idea of attacking Tunny by machine was worth its salt and that
Tutte’s method succeeded in practice. However, Heath Robinson
suffered from ‘intolerable handicaps’.42
Despite the high speed of the electronic counters, Heath Robinson was
not really fast enough for the codebreakers’ requirements,
taking several hours to elucidate a single message.43
Moreover, the counters were not fully reliable—Heath Robinson
was prone to deliver different results if set the same problem twice.
Mistakes made in hand-punching the two tapes were another fertile
source of error, the long chi-tape being especially difficult to
prepare. At first, undetected tape errors prevented Heath Robinson
from obtaining any results at all.44
And paramount among the difficulties was that the two tapes would get
out of synchronisation with each other as they span, throwing the
calculations out completely. The loss of synchronisation was caused
by the tapes stretching, and also by uneven wear around the sprocket
holes.

The question was how to build a better machine—a question for
an engineer. In a stroke of genius, the electronics expert Thomas
Flowers solved all these problems.

Flowers, neglected pioneer of computing

During the
1930s Flowers pioneered the large-scale use of electronic valves to
control the making and breaking of telephone connections.45
He was swimming against the current. Many regarded the idea of
large-scale electronic equipment with scepticism. The common wisdom
was that valves—which, like light bulbs, contained a hot
glowing filament—could never be used satisfactorily in large
numbers, for they were unreliable, and in a large installation too
many would fail in too short a time. However, this opinion was based
on experience with equipment that was switched on and off
frequently—radio receivers, radar, and the like. What Flowers
discovered was that, so long as valves were switched on and left on,
they could operate reliably for very long periods, especially if
their ‘heaters’ were run on a reduced current.

At that time, telephone switchboard equipment was based on the relay.
A relay is a small, automatic switch. It contains a mechanical
contact-breaker—a moving metal rod that opens and closes an
electrical circuit. The rod is moved from the ‘off’
position to the ‘on’ position by a magnetic field. A
current in a coil is used to produce the magnetic field; as soon as
the current flows, the field moves the rod. When the current ceases,
a spring pushes the rod back to the ‘off’ position.
Flowers recognised that equipment based instead on the electronic
valve—whose only moving part is a beam of electrons—not
only had the potential to operate very much faster than relay-based
equipment, but was in fact potentially more reliable, since valves
are not prone to mechanical wear.

In 1934 Flowers wired together an experimental installation
containing three to four thousand valves (by contrast, Wynn-Williams’
electronic counters of 1931 contained only three or four valves).
This equipment was for controlling connections between telephone
exchanges by means of tones, like today’s touch-tones (a
thousand telephone lines were controlled, each line having 3-4 valves
attached to its end). Flowers’ design was accepted by the Post
Office and the equipment went into limited operation in 1939. Flowers
had proved that an installation containing thousands of valves would
operate very reliably—but this equipment was a far cry from
Colossus. The handful of valves attached to each telephone line
formed a simple unit, operating independently of the other valves in
the installation, whereas in Colossus large numbers of valves worked
in concert.

During the same period before the war Flowers explored the idea of
using valves as high-speed switches. Valves were used originally for
purposes such as amplifying radio signals. The output would vary
continuously in proportion to a continuously varying input, for
example a signal representing speech. Digital computation imposes
different requirements. What is needed for the purpose of
representing the two binary digits, 1 and 0, is not a continuously
varying signal but plain ‘on’ and ‘off’ (or
‘high’ and ‘low’). It was the novel idea of
using the valve as a very fast switch, producing pulses of current
(pulse for 1, no pulse for 0) that was the route to high-speed
digital computation. During 1938-9 Flowers worked on an experimental
high-speed electronic data store embodying this idea. The store was
intended to replace relay-based data stores in telephone exchanges.
Flowers’ long-term goal was that electronic equipment should
replace all the relay-based systems in telephone exchanges.

By the time of the outbreak of war with Germany, only a small number
of electrical engineers were familiar with the use of valves as
high-speed digital switches. Thanks to his pre-war research, Flowers
was (as he himself remarked) possibly the only person in Britain who
realized that valves could be used reliably on a large scale for
high-speed digital computing.46
When Flowers was summoned to Bletchley Park—ironically, because
of his knowledge of relays—he turned out to be the right man in
the right place at the right time.

Turing, working on Enigma, had approached Dollis Hill to build a
relay-based decoding machine to operate in conjunction with the Bombe
(the Bombe itself was also relay-based). Once the Bombe had uncovered
the Enigma settings used to encrypt a particular message, these
settings were to be transferred to the machine requisitioned by
Turing, which would automatically decipher the message and print out
the German plaintext.47
Dollis Hill sent Flowers to Bletchley Park. He would soon become one
of the great figures of World War II codebreaking. In the end, the
machine Flowers built for Turing was not used, but Turing was
impressed with Flowers, who began thinking about an electronic Bombe,
although he did not get far. When the teleprinter group at Dollis
Hill ran into difficulties with the design of the Heath Robinson’s
combining unit, Turing suggested that Flowers be called in. (Flowers
was head of the switching group at Dollis Hill, located in the same
building as the teleprinter group.) Flowers and his switching group
improved the design of the combining unit and manufactured it.48

Flowers did not think much of the Robinson, however. The basic design
had been settled before he was called in and he was sceptical as soon
as Morrell, head of the teleprinter group, first told him about it.
The difficulty of keeping two paper tapes in synchronisation at high
speed was a conspicuous weakness. So was the use of a mixture of
valves and relays in the counters, because the relays slowed
everything down: Heath Robinson was built mainly from relays and
contained no more than a couple of dozen valves. Flowers doubted that
the Robinson would work properly and in February 1943 he presented
Newman with the alternative of a fully electronic machine able to
generate the chi-stream (and psi- and motor-streams) internally.49

Flowers’ suggestion was received with ‘incredulity’
at TRE and Bletchley Park.50
It was thought that a machine containing the number of valves that
Flowers was proposing (between one and two thousand) ‘would be
too unreliable to do useful work’.51
In any case, there was the question of how long the development
process would take—it was felt that the war might be over
before Flowers’ machine was finished. Newman pressed ahead with
the two-tape machine. He offered Flowers some encouragement but
effectively left him to do as he wished with his proposal for an
all-electronic machine. Once Heath Robinson was a going concern,
Newman placed an order with the Post Office for a dozen more
relay-based two-tape machines (it being clear, given the quantity and
very high importance of Tunny traffic, that one or two machines would
not be anywhere near enough).Meanwhile Flowers, on his own
initiative and working independently at Dollis Hill, began building
the fully electronic machine that he could see was necessary. He
embarked on Colossus, he said, ‘in the face of scepticism’52
from Bletchley Park and ‘without the concurrence of BP’.53
‘BP weren’t interested until they saw it [Colossus]
working’, he recollected.54
Fortunately, the Director of the Dollis Hill Research Station,
Gordon Radley, had greater faith in Flowers and his ideas, and placed ‘the
whole resources of the laboratories’ at Flowers’
disposal.55

Colossus

The
prototype Colossus was brought to Bletchley Park in lorries and
reassembled by Flowers’ engineers.56
It had approximately 1600 electronic valves and operated at 5000
characters per second. Later models, containing approximately 2400
valves, processed five streams of dot-and-cross simultaneously, in
parallel. This boosted the speed to 25,000 characters per second.
Colossus generated the chi-stream electronically. Only one tape was
required, containing the ciphertext—the synchronisation problem
vanished. (Flowers’ original plan was to dispense with the message
tape as well and set up the ciphertext, as well as the wheels, on
valves; but he abandoned this idea when it became clear that messages
of 5000 or more characters would have to be processed.57)

The arrival of the prototype Colossus caused quite a stir. Flowers
said:

I don’t think they [Newman et al.] really understood what I was
saying in detail — I am sure they didn’t — because when
the first machine was constructed and working, they obviously were
taken aback. They just couldn’t believe it! ... I don’t
think they understood very clearly what I was proposing until they
actually had the machine.58

On what date did Colossus first come alive? In his written and
verbal recollections Flowers was always definite that Colossus was
working at Bletchley Park in the early part of December 1943.59
In three separate interviews he recalled a key date quite
specifically, saying that Colossus carried out its first trial run at
Bletchley Park on 8 December 1943.60
However, Flowers’ personal diary for 1944—not discovered
until after his death—in fact records that Colossus did not
make the journey from Dollis Hill to Bletchley Park until January
1944. On Sunday 16 January Colossus was still in Flowers’ lab
at Dollis Hill. His diary entry shows that Colossus was certainly
working on that day. Flowers was busy with the machine from the
morning until late in the evening and he slept at the lab.

Flowers’ entry for 18 January reads simply: ‘Colossus
delivered to B.P.’. This is confirmed by a memo dated 18
January from Newman to Travis (declassified only in 2004). Newman
wrote ‘Colossus arrives to-day’.61
Colossus cannot therefore have carried out its first trial run at
Bletchley Park in early December. What did happen on 8 December 1943,
the date that stuck so firmly in Flowers’ mind? Perhaps this
was indeed the day that Colossus processed its first test tape at
Dollis Hill. ‘I seem to recall it was in December’, says Harry
Fensom, one of Flowers’ engineers.62

By
February 1944 the engineers had got Colossus ready to begin serious
work for the Newmanry. Tutte’s statistical method could now be used
at electronic speed. The computer attacked its first message on
Saturday 5 February. Flowers was present. He noted laconically in his
diary, ‘Colossus did its first job. Car broke down on way
home.’

Colossus immediately doubled the codebreakers’ output.63
The advantages of Colossus over Robinson were not only its greatly
superior speed and the absence of synchronised tapes, but also its
greater reliability, resulting from Flowers’ redesigned
counters and the use of valves in place of relays throughout. It was
clear to the Bletchley Park authorities—whose scepticism was
now completely cured—that more Colossi were required urgently.

Indeed, a crisis had developed, making the work of Newman’s
section even more important than before. Since the German
introduction of the QEP system in October 1942, the codebreakers
using hand-methods to crack Tunny messages had been reliant upon
depths, and as depths became rarer during 1943, the number of broken
messages reduced to a trickle.64
Then things went from bad to worse. In December 1943 the Germans
started to make widespread use of an additional device in the Tunny
machine, whose effect was to make depth-reading impossible (by
allowing letters of the plaintext itself to play a role in the
generation of the key). The hand breakers had been prone to scoff at
the weird contraptions in the Newmanry, but suddenly Newman’s
machines were essential
to all Tunny work.65

In March
1944 the authorities demanded four more Colossi. By April they were
demanding twelve.66
Great pressure was put on Flowers to deliver the new machines
quickly. The instructions he received came ‘from the highest
level’—the War Cabinet—and he caused consternation
when he said flatly that it was impossible to produce more than one
new machine by 1 June 1944.67

Flowers had managed to produce the prototype Colossus at Dollis Hill
only because many of his laboratory staff ‘did nothing but
work, eat, and sleep for weeks and months on end’.68 He needed greater production capacity, and proposed to take
over a Post Office factory in Birmingham. Final assembly and testing
of the computers would be done at his Dollis Hill laboratory. Flowers
estimated that once the factory was in operation he would be able to
produce additional Colossi at the rate of about one per month.69 He recalled how one day some Bletchley people
came to inspect the work, thinking that Flowers might be
‘dilly-dallying’: they returned ‘staggered at the
scale of the effort’.70
Churchill for his part gave Flowers top priority for everything he
needed.71

By means of repluggable cables and panels of switches, Flowers
deliberately built more flexibility than was strictly necessary into
the logic units of the prototype Colossus. As a result, new methods
could be implemented on Colossus as they were discovered. In February
1944 two members of the Newmanry, Donald Michie and Jack Good, had
quickly found a way of using Colossus to discover the Tunny wheel
patterns.72
Flowers was told to incorporate a special panel for breaking wheel
patterns in Colossus II.

Colossus II—the first of what Flowers referred to as the ‘Mark
2’ Colossi73—was
shipped from Dollis Hill to Bletchley Park on 4 May 1944.74
The plan was to assemble and test Colossus II at Bletchley Park
rather than Dollis Hill, so saving some precious time.75 Promised by the first of June, Colossus II was still not
working properly as the final hours of May ticked past. The computer
was plagued by intermittent and mysterious faults.76 Flowers struggled to find the problem, but midnight came and
went. Exhausted, Flowers and his team dispersed at 1 am to snatch a
few hours sleep.77
They left Chandler to work on, since the problem appeared to be in a
part of the computer that he had designed. It was a tough night:
around 3 am Chandler noticed that his feet were getting wet.78
A radiator pipe along the wall had sprung a leak, sending a dangerous
pool of water towards Colossus.

Flowers returned to find the computer running perfectly. ‘Colossus
2 in operation’, he noted in his diary.79
The puddle remained, however, and the women operators had to don
gumboots to insulate themselves.80
During the small hours Chandler had finally tracked down the fault in
Colossus (parasitic oscillations in some of the valves) and had fixed
it by wiring in a few extra resistors.81 Flowers
and his ‘band of brothers’ had met BP’s
deadline—a deadline whose significance Flowers can only have
guessed at.82

Less than a week later the Allied invasion of France began. The D-day
landings of June 6 placed huge quantities of men and equipment on the
beaches of Normandy. From the beachheads the Allies pushed their way
into France through the heavy German defences. By mid-July the front
had advanced only 20 or so miles inland, but by September Allied
troops had swept across France and Belgium and were gathering close
to the borders of Germany, on a front extending from Holland in the
north to Switzerland in the south.83

Since the early months of 1944, Colossus I had been providing an
unparalleled window on German preparations for the Allied invasion.84
Decrypts also revealed German appreciations of Allied intentions.
Tunny messages supplied vital confirmation that the German planners
were being taken in by Operation Fortitude, the extensive
programme of deceptive measures designed to suggest that the invasion
would come further north, in the Pas de Calais.85
In the weeks following the start of the invasion the Germans
tightened Tunny security, instructing operators to change the
patterns of the chi- and psi-wheels daily instead of monthly. Hand
methods for discovering the new patterns were overwhelmed. With
impeccable timing Colossus II’s device for breaking wheel
patterns came to the rescue.

Once Flowers’ factory in Birmingham was properly up and
running, new Colossi began arriving in the Newmanry at roughly six
week intervals. Eventually three were dedicated to breaking wheel
patterns.86
Flowers was a regular visitor at B.P. throughout the rest of 1944,
overseeing the installation programme for the Mark 2 Colossi.87
By the end of the year seven Colossi were in operation. They provided
the codebreakers with the capacity to find all twelve wheel settings
by machine, and this was done in the case of a large proportion of
decrypted messages.88
There were ten Colossi in operation by the time of the German
surrender in 1945, and an eleventh was almost ready.

Misconceptions about Colossus

One of the
most common misconceptions in the secondary literature is that
Colossus was used against Enigma. Another is that Colossus was used
against not Tunny but Sturgeon—an error promulgated by Brian
Johnson’s influential televison series and accompanying book The
Secret War.89 There are in fact many wild
tales about Colossus in the history books. GeorgesIfrah even
states that Colossus produced English plaintext from the
German ciphertext!90 As already explained, the
output of Colossus was a series of counts indicating the correct
wheel settings (or, later, the wheel patterns). Not even the de-chi
was produced by Colossus itself, let alone the plaintext—and
there was certainly no facility for the automatic translation of
German into English.

J. A. N. Lee (in his book Computer Pioneers): ‘Newman
fully appreciated the significance of Turing’s ideas for the design
of high-speed electronic machines for searching for wheel patterns
and placings on the highest-grade German enciphering machines, and
the result was the invention of the "Colossus"’.56

Time
magazine reported, in total confusion:

‘At Bletchley Park, Alan Turing built a succession of
vacuum-tube machines called Colossus that made mincemeat of Hitler’s
Enigma codes’ (March 29, 1999).57

An
insidious misconception concerns ownership of the inspiration for
Colossus. Many accounts identify Turing as the key figure in the
designing of Colossus. In a biographical article on Turing, the
computer historian J. A. N. Lee said that Turing’s ‘influence on the
development of Colossus is well known’,91 and in an article
on Flowers, Lee referred to Colossus as ‘the cryptanalytical machine
designed by Alan Turing and others’.92
Even a book on sale at the Bletchley Park Museum states that at
Bletchley Park ‘Turing worked ... on what we now know was computer
research’ which led to ‘the world’s first electronic, programmable
computer, "Colossus"’.93

The view
that Turing’s interest in electronics contributed to the inspiration
for Colossus is indeed common. This claim is enshrined in
codebreaking exhibits in leading museums; and in the Annals of the
History of Computing Lee and Holtzman state that Turing
‘conceived of the construction and usage of high-speed electronic
devices; these ideas were implemented as the "Colossus"
machines’.94 However, the definitive 1945
General Report on Tunny makes matters perfectly clear:
‘Colossus was entirely the idea of Mr. Flowers’ (see the extract from page 35 in the right-hand column).95 By 1943
electronics had been Flowers’ driving passion for more than a decade
and he needed no help from Turing. Turing was, in any case, away in
the United States during the critical period at the beginning of 1943
when Flowers proposed his idea to Newman and worked out the design of
Colossus on paper. Flowers emphasised in an interview that Turing
‘made no contribution’ to the design of Colossus.96
Flowers said: ‘I invented the Colossus. No one else was capable of
doing it.’97

Martin Davis (in The Universal Computer: The Road from Leibniz to Turing): Some of the methods ... used were playfully called turingismus
indicating their source. But turingismus required the processing of
lots of data and for the decryption be [sic] of any use, the
processing had to be done very quickly. ... In March 1943, Alan
Turing sailed home from a visit of several months in the United
States ... He whiled away the time during his Atlantic passage by
studying [an] RCA catalog, for it had been found that vacuum tubes
could carry out the kind of logical switching previously done by
electric relays. And the tubes were fast ... Vacuum tube circuits had
in fact been used experimentally for telephone switching, and Turing
had made contact with the gifted engineer, T. Flowers, who had
spearheaded this research. Under the direction of Flowers and Newman,
a machine, essentially a physical embodiment of turingismus, was
rapidly brought into being. Dubbed the Colossus and an engineering
marvel, this machine contained 1500 vacuum tubes.59

In his
recent book on the history of computing, Martin Davis offers a garbled account of Colossus (see right-hand column).
Here Davis conflates Turingery, which he calls ‘turingismus’, with
Tutte’s statistical method. (ismus is a German suffix
equivalent to the English ism. Newmanry codebreaker Michie
explains the origin of Turingery’s slang name ‘Turingismus’: ‘three
of us (Peter Ericsson, Peter Hilton and I) coined and used in playful
style various fake-German slang terms for everything under the sun,
including occasionally something encountered in the working
environment. Turingismus was a case of the latter.’98)
Turing’s method of wheel breaking from depths and Tutte’s method of
wheel setting from non-depths were distant relatives, in that both
used delta-ing. But there the similarity ended. Turingery, Tutte
said, seemed to him ‘more artistic than mathematical’; in applying
the method you had to rely on what ‘you felt in your bones’.99 Conflating the two methods, Davis erroneously concludes
that Colossus was a physical embodiment of Turingery. But as explained above,
Turingery was a hand method—it was Tutte’s method that
‘required the processing of lots of data’. Tutte’s method, not
Turingery, was implemented in Heath Robinson and Colossus. ‘Turingery
was not used in either breaking or setting by any valve machine of
any kind’, Michie underlined.100

The first stored-program electronic computer, built by Tom Kilburn (left) and Freddie Williams (right) in Newman’s Computing Machine Laboratory at the University of Manchester.74

The pilot model of Turing’s Automatic Computing Engine, the fastest of the early machines and precursor of the DEUCE computers.75

If Flowers
could have patented the inventions that he contributed to the assault
on Tunny, he would probably have become a very rich man. As it was,
the personal costs that he incurred in the course of building the
Colossi left his bank account overdrawn at the end of the war. Newman
was offered an OBE for his contribution to the defeat of Germany, but
he turned it down, remarking to ex-colleagues from Bletchley Park
that he considered the offer derisory.101
Tutte received no public recognition for his vital work. Turing
accepted an OBE, which he kept in his toolbox.

At the
end of hostilities, orders were received from Churchill to break up
the Colossi, and all involved with Colossus and the cracking of Tunny
were gagged by the Official Secrets Act. The very existence of
Colossus was to be classified indefinitely. Flowers described his
reactions:

When after the war
ended I was told that the secret of Colossus was to be kept
indefinitely I was naturally disappointed. I was in no doubt, once it
was a proven success, that Colossus was an historic breakthrough, and
that publication would have made my name in scientific and
engineering circles—a conviction confirmed by the reception
accorded to ENIAC, the U.S. equivalent made public just after the war
ended. I had to endure all the acclaim given to that enterprise
without being able to disclose that I had anticipated it. What I lost
in personal prestige, and the benefits which commonly accrue in such
circumstances, can now only be imagined. But at the time I accepted
the situation philosophically and, in the euphoria of a war that was
won, lost any concern about what might happen in the future.102

ENIAC, commissioned by the U.S. army in 1943, was designed to calculate
trajectories of artillery shells. Although not operational until the
end of 1945—two years after Colossus first ran—ENIAC is
standardly described as the first electronic digital computer.
Flowers’ view of the ENIAC? It was just a number
cruncher—Colossus, with its elaborate facilities for logical
operations, was ‘much more of a computer than ENIAC’.103

The Newmanry’s Colossi might have passed into the public domain at
the end of the fighting, to become, like ENIAC, the electronic muscle
of a scientific research facility. The Newmanry’s engineers would
quickly have adapted the equipment for peacetime applications. The
story of computing might have unfolded rather differently with such a
momentous push right at the beginning. Churchill’s order to destroy
the Colossi was an almighty blow in the face for science—and
for British industry.

In April 1946, codebreaking operations were transferred from
Bletchley Park to buildings in Eastcote in suburban London.104
At the time of the move, the old name of the organisation,
‘Government Code and Cypher School’, was formally changed to
‘Government Communications Headquarters’ (GCHQ).105
Six years later another move commenced, and during 1952-54 GCHQ
shifted its personnel and equipment, including its codebreaking
machinery, away from the London area to a large site in Cheltenham.106
Some machines did survive the dissolution of the Newmanry. Two
Colossi made the move from Bletchley Park to Eastcote, and then
eventually on to Cheltenham.107
They were accompanied by two of the replica Tunny
machines manufactured at Dollis Hill.108
One of the Colossi, known as ‘Colossus Blue’ at GCHQ, was dismantled
in 1959 after fourteen years of postwar service. The remaining
Colossus is believed to have stopped running in 1960.

During their later years the two Colossi were used extensively for
training. Details of what they were used for prior to this remain
classified. There is a hint of the importance of one new role for
these Newmanry survivors in a letter written by Jack Good:

I heard that Churchill requested that all
Colossi be destroyed after the war, but GCHQ decided to keep at least
one of them. I know of that one because I used it myself. That was
the first time it was used after the war. I used it for a purpose for
which NSA [National Security Agency]
were planning to build a new special-purpose machine. When I showed
that the job could be carried out on Colossus, NSA decided not to go
ahead with their plan. That presumably is one reason I am still held
in high regard in NSA. Golde told me that one of his friends who
visits NSA told Golde that I am ‘regarded as God’ there.109

After
Bletchley’s own spectacular successes against the German machines,
GCHQ was—not unnaturally—reluctant to use key-generating
cipher machines to protect British high-grade diplomatic traffic.
Instead GCHQ turned to one-time pad. Sender and receiver were issued
with identical key in the form of a roll of teleprinter tape. This
would be used for one message only. One-time pad is highly secure.
The disadvantage is that a complex and highly efficient distribution
network is required to supply users with key. It is probably true
that GCHQ initially underestimated the difficulties of distributing
key.

The GCHQ Colossi assisted in the
production of one-time pad. Ex-Newmanry engineers used some of
Flowers’ circuitry from Colossus to build a random noise generator
able to produce random teleprinter characters on a punched tape. This
device, code-named ‘Donald Duck’, exploited the random way in which
electrons are emitted from a hot cathode. The tapes produced by
Donald Duck were potential one-time pad. The tapes were checked by
Colossus, and those that were not flat-random were weeded out.
Newmanry-type tape-copying machines were used to make copies of tapes
that passed the tests, and these were distributed to GCHQ’s clients.

Probably the Colossi had additional
postwar applications. They may have been used to make character
counts of enemy cipher traffic, searching for features that might
give the cryptanalysts a purchase. Perhaps the GCHQ Colossi were even
used against reconditioned German Tunny machines. Many Tunnies were
captured by the invading British armies during the last stages of the
war. If the National interest so dictated, Tunny machines may have
been sold to commercial organisations or foreign powers, and the
resulting traffic read by GCHQ.

Until the 1970s few had any idea that electronic computation had been
used successfully during the Second World War. In 1975, the British
government released a set of captioned photographs of the Colossi (several of which are reproduced above).110
By 1983, Flowers had received clearance to publish an account of the
hardware of the first Colossus.111 Details of the later
Colossi remained secret. So, even more importantly, did all
information about how Flowers’ computing machinery was actually
used by the codebreakers. Flowers was told by the British authorities
that ‘the technical description of machines such as COLOSSUS
may be disclosed’, but that he must not disclose any
information about ‘the functions which they performed’.112
It was rather like being told that he could give a detailed technical
description of the insides of a radar receiver, but must not say
anything about what the equipment did (in the case of radar, reveal
the location of planes, submarines, etc., by picking up radio waves
bouncing off them). He was also allowed to describe some aspects of
Tunny, but there was a blanket prohibition on saying anything at all
relating to ‘the weaknesses which led to our successes’.
In fact, a clandestine censor objected to parts of the account that
Flowers wrote, and he was instructed to remove these prior to
publication.113

There matters more or less stood until 1996, when the U.S. Government
declassified some wartime documents describing the function of
Colossus. These had been sent to Washington during the war by U.S.
liaison officers stationed at Bletchley Park. The most important
document remained classified, however: the 500 page General Report
on Tunny written at Bletchley Park in 1945 by Jack Good, Donald
Michie, and Geoffrey Timms. Thanks largely to Michie’s tireless
campaigning, the report was declassified by the British Government in
June 2000, finally ending the secrecy.

Colossus and the modern computer

As
everyone who can operate a personal computer knows, the way to make
the machine perform the task you want—word-processing, say—is
to open the appropriate program stored in the computer’s
memory. Life was not always so simple. Colossus did not store
programs in its memory. To set up Colossus for a different job, it
was necessary to modify some of the machine’s wiring by hand,
using switches and plugs. The larger ENIAC was also programmed by
re-routing cables and setting switches. The process was a nightmare:
it could take the ENIAC’s operators up to three weeks to set up
and debug a program.114 Colossus, ENIAC, and
their like are called ‘program-controlled’ computers, in
order to distinguish them from the modern ‘stored-program’
computer.

This basic principle of the modern
computer, that is, controlling the machine’s operations by
means of a program of coded instructions stored in the computer’s
memory, was thought of by Turing in 1936. At the time, Turing was a
shy, eccentric student at Cambridge University. His ‘universal
computing machine’, as he called it—it would soon be
known simply as the universal Turing machine—emerged from
research that no-one would have guessed could have any practical
application. Turing was working on a problem in mathematical logic,
the so-called ‘decision problem’, which he learned of
from lectures given by Newman. (For a description of the decision
problem and Turing’s approach to it, see ‘Computable Numbers: A Guide’ in The Essential Turing.115) In
the course of his attack on this problem, Turing thought up an
abstract digital computing machine which, as he said, could compute
‘all numbers which could naturally be regarded as computable’.116 The universal Turing machine consists of a
limitless memory in which both data and instructions are stored, in
symbolically encoded form, and a scanner that moves back and forth
through the memory, symbol by symbol, reading what it finds and
writing further symbols. By inserting different programs into the
memory, the machine can be made to carry out any algorithmic task.
That is why Turing called the machine universal.

Turing’s fabulous idea was just this: a single machine of fixed
structure that, by making use of coded instructions stored in memory,
could change itself, chameleon-like, from a machine dedicated to one
task into a machine dedicated to a completely different task—from
calculator to word processor, for example. Nowadays, when many have a
physical realisation of a universal Turing machine in their living
room, this idea of a one-stop-shop computing machine is apt to seem
as obvious as the wheel. But in 1936, when engineers thought in terms
of building different machines for different purposes, the concept of
the stored-program universal computer was revolutionary.

In 1936 the universal Turing machine existed only as an idea. Right
from the start Turing was interested in the possibility of building
such a machine,
as to some extent was Newman, but before the war they knew of no
practical way to construct a stored-program computer.117 It was not
until the advent of Colossus that the dream of building an
all-purpose electronic computing machine took hold of them. Flowers
had established decisively and for the first time that large-scale
electronic computing machinery was practicable, and soon after the
end of the war Turing and Newman both embarked on separate projects
to create a universal Turing machine in hardware. Racks of electronic
components from the dismantled Colossi were shipped from Bletchley
Park to Newman’s Computing Machine Laboratory at Manchester.
Historians who did not know of Colossus tended to assume quite
wrongly that Turing and Newman inherited their vision of an
electronic computer from the ENIAC group in the U.S.

Even in
the midst of the attack on Tunny, Newman was thinking about the
universal Turing machine. He showed Flowers Turing’s 1936 paper
about the universal machine, ‘On Computable Numbers’,
with its key idea of storing symbolically encoded instructions in
memory, but Flowers, not being a mathematical logician, ‘didn’t
really understand much of it’.118 There is little doubt that by 1944 Newman had firmly in mind the
possibility of building a universal Turing machine using electronic
technology. It was just a question of waiting until he ‘got
out’.119 In February 1946, a few months after his appointment to the
University of Manchester, Newman wrote to the Hungarian-American
mathematician von Neumann (like Newman considerably influenced by
Turing’s 1936 paper, and himself playing a leading role in the
post-ENIAC developments taking place in the U.S.):

I am
... hoping to embark on a computing machine section here, having got
very interested in electronic devices of this kind during the last
two or three years. By about eighteen months ago I had decided to try
my hand at starting up a machine unit when I got out. ... I am of
course in close touch with Turing.120

The implication of Flowers’ racks of electronic equipment was
obvious to Turing too. Flowers said that once Colossus was in
operation, it was just a matter of Turing’s waiting to see what
opportunity might arise to put the idea of his universal computing
machine into practice. (By the end of the war, Turing had educated
himself thoroughly in electronic engineering: during the later part
of the war he gave a series of evening lectures ‘on valve
theory’.121) Turing’s opportunity came
along in 1945, when John Womersley, head of the Mathematics Division
of the National Physical Laboratory (NPL) in London, invited him to
design and develop an electronic stored-program digital computer.
Turing’s technical report ‘Proposed Electronic
Calculator’,122 dating from the end of 1945
and containing his design for the ACE, was the first relatively
complete specification of an electronic stored-program digital
computer.123
The slightly earlier ‘First Draft of a Report on the EDVAC’,124 produced in about May 1945 by von Neumann, was
much more abstract, saying little about programming, hardware
details, or electronics. (The EDVAC, proposed successor to the ENIAC,
was to be a stored-program machine. It was not fully working until
1952.125) Harry Huskey, the electronic engineer who
subsequently drew up the first detailed hardware designs for the
EDVAC, stated that the ‘information in the “First Draft”
was of no help’.126
Turing, in contrast, supplied detailed circuit designs, full
specifications of hardware units, specimen programs in machine code,
and even an estimate of the cost of building the machine.

Turing asked Flowers to build the ACE, and in March 1946 Flowers said
that a ‘minimal ACE’ would be ready by August or
September of that year.127
Unfortunately, however, Dollis Hill was overwhelmed by a backlog of
urgent work on the national telephone system, and it proved
impossible to keep to Flowers’ timetable. In the end it was
Newman’s team who, in June 1948, won the race to build the first
stored-program computer. The first program, stored on the face of a
cathode ray tube as a pattern of dots, was inserted manually, digit
by digit, using a panel of switches. The news that the Manchester
machine had run what was only a tiny program—just 17
instructions long—for a mathematically trivial task was
‘greeted with hilarity’ by Turing’s team working on
the much more sophisticated ACE.128

A pilot
model of the ACE ran its first program in May 1950. With an operating
speed of 1 MHz, the pilot model ACE was for some time the fastest
computer in the world. The pilot model was the basis for the very
successful DEUCE computers, which became a cornerstone of the
fledgling British computer industry—confounding the suggestion,
made in 1946 by Sir Charles Darwin, Director of the NPL and grandson
of the great Darwin, that ‘it is very possible that ... one
machine would suffice to solve all the problems that are demanded of
it from the whole country’.129

In teleprinter code the letters most frequently used
are represented by the fewest holes in the tape, which is to say by
the fewest crosses, in B.P. notation.130 For instance E, the commonest
letter of English, is x••••,
and T, the next most frequent, is ••••x.
The table in the right-hand column gives the 5-bit teleprinter code for each
character of the teleprint alphabet.

The left-hand column of the table shows the characters of the
teleprint alphabet as they would have been written down by the
Bletchley codebreakers. For example, the codebreakers wrote ‘9’ to
indicate a space (as in ‘to9indicate’) and ‘3’ to indicate a carriage
return.

The ‘move to figure shift’ character (which some at Bletchley wrote
as ‘+’ and some as ‘5’) told the teleprinter to shift from printing
letters to printing figures; and the ‘move to letter shift’ character
(written ‘−’ or ‘8’) told the
machine to shift from printing figures to printing letters. With the
teleprinter in letter mode, the keys along the top row of the
keyboard would print QWERTYUIOP, and in figure
mode the same keys would print 1234567890.

Most of
the keyboard characters had different meanings in letter mode and
figure mode. In figure mode the M-key printed a full stop, the N-key
a comma, the C-key a colon, and the A-key a dash, for example.
(Unlike a modern keyboard, the teleprinter did not have separate keys
for punctuation.) The meanings of the other keys in figure mode are
given at the right of the table.

To cause
the teleprinter to print 123WHO,
ME? the operator must first press figure shift and key Q W E
to produce the numbers. He or she then drops into letter mode and
keys a space (or vice versa), followed by W H O. To produce the
comma it is necessary to press figure shift then N. This is followed
by letter shift, space, and M E. A final figure shift followed by B
produces the question mark.

+QWE−9WHO+N−9ME+B

Often Tunny operators would repeat the figure-shift and letter-shift
characters, sending a comma as ++N−− and a full stop as ++M−−, for
example. Following this practice, the operator would key

++QWE−−9WHO++N−−9ME++B

Presumably the shift characters were repeated to ensure that the
shift had ‘taken’. These repetitions were very helpful to the
British, since a correct guess at a punctuation mark could yield six
characters of text (including the trailing 9).

First, some notation. P
is the plaintext, C is the cipher text, χ
is the stream of letters contributed to the message’s key by the
chi-wheels, and Ψ
is the stream contributed by the psi-wheels. χ
+ Ψ is the result of adding
χ and Ψ
using the rules of Tunny-addition. ΔC
is the result of delta-ing the ciphertext, Δ(χ
+ Ψ) is the
result of delta-ing the stream of characters that results from adding
χ and Ψ,
and so forth.

Since C is produced
by adding the key to P, and the key is produced by adding χ
and Ψ, the fundamental encipherment
equation for the Tunny machine is:

C = P + χ
+ Ψ

C1
is written for the first impulse of C (i.e. the first of the
five streams in the teleprint representation of the ciphertext); and
similarly P1, χ1
and Ψ1 are the first
impulses of P, χ and Ψ
respectively. The encipherment equation for the first impulse is:

C1 = P1 + χ1
+ Ψ1

Delta-ing each side of this equation gives

ΔC1 = Δ(P1
+ χ1 + Ψ1)

Delta-ing the sum of two or
more impulses produces the same result as first delta-ingeach
impulse and then summing. So

ΔC1 = ΔP1
+ Δχ1 + ΔΨ1

Likewise for the second impulse:

ΔC2 = ΔP2
+ Δχ2 + ΔΨ2

Adding
the equations for the first and second impulses gives

ΔC1 + ΔC2
= ΔP1 + ΔP2
+ Δχ1 + Δχ2
+ ΔΨ1 + ΔΨ2

which
is the same as

Δ(C1 + C2)
= Δ(P1 + P2)
+ Δ(χ1
+ χ2)+
Δ(Ψ1
+ Ψ2)

Because of the staggering
motion of the psi-wheels, Δ(Ψ1
+ Ψ2)turns out to be about 70% dot. But
adding dot leaves you where you started: cross plus dot is dot and
dot plus dot is dot. It follows that the addition of Δ(Ψ1
+ Ψ2) more
often than not has no effect. So

Δ(C1 + C2)
= Δ(P1 + P2)
+ Δ(χ1
+ χ2)

is
true more often than not.

Tutte also discovered that
Δ(P1 + P2) is approximately 60% dot. This effect is
the result of various factors, for instance the Tunny operators’
habit of repeating certain characters (see Appendix 1), and
contingencies of the way the individual letters are represented in
the underlying teleprinter code—for example, the delta of the
sum of the first and second impulses of the common bigram (or letter
pair) DE is dot, as it is for other common bigrams such as BE, ZE,
ES. So it is true more often than not that

Δ(C1 + C2)
= Δ(χ1
+ χ2)

Tutte’s ‘1 + 2 break in’ is this. Δ(C1
+ C2) is
stepped through the delta-ed sum of the first and second impulses of
the entire stream of characters from the chi-wheels. Generally the
correspondence between Δ(C1
+ C2)and a strip from the delta-ed chi of the
same length will be no better than chance. If, however, Δ(C1
+ C2) and a strip of delta-ed chi correspond
more often than not, then a candidate has been found for Δ(χ1
+ χ2), and so for
the first two impulses of χ.
The greater the correspondence, the likelier the candidate.131

References

[1] Bauer, F. L. 2006 ‘The Tiltman Break’, in [10].

[2] Cairncross, J. 1997 The Enigma Spy: The
Story of the Man who Changed the Course of World War Two, London:
Century.

[3] Campbell-Kelly, M. 2005 ‘The ACE and the Shaping of British
Computing’, in [9].

1
The physical Tunny machine is described in section 11 of General
Report on Tunny, and in Davies [11]. The machine’s function and
use is described in sections 11 and 94 of General Report on
Tunny. General Report on Tunny was written at Bletchley
Park in 1945 by Tunny-breakers Jack Good, Donald Michie and Geoffrey
Timms; it was released by the British government in 2000 to the
National Archives/Public Record Office (PRO) at Kew (document
reference HW 25/4 (vol. 1), HW 25/5 (vol. 2)). A digital facsimile
is available in The Turing Archive for the History of Computing
http://www.AlanTuring.net/tunny_report.

16
British message reference number CX/MSS/2499/T14;
PRO reference HW1/1648. Words enclosed in square brackets do not
appear in the original. (Thanks to Ralph Erskine for assistance in
locating this document. An inaccurate version of the intercept
appears in Hinsley [19], pp. 764-5.)

19
Documents from G.C. & C.S. to Churchill, 30
April 1943 (PRO reference HW1/1648). An earlier decrypt concerning
Zitadelle (13 April 1943), and an accompanying note from ‘C’
to Churchill, are at HW1/1606.

127
’Status of the Delay Line Computing Machine at the P.O. Research
Station’ (anon., National Physical Laboratory, 7 March 1946; in the
Woodger Papers (catalogue reference M12/105); a digital facsimile is
in The Turing Archive for the History of Computing
http://www.AlanTuring.net/delay_line_status).

131 This article is a revised and illustrated version of Copeland, B.J. ‘Breaking the Lorenz Schlüsselzusatz Traffic’, in de Leeuw, K., Bergstra, J. (eds) The History of Information Security: A Comprehensive Handbook (Amsterdam: Elsevier Science, 2007), pp. 447-477.